It was a mistake to exclude Russia from the G8 in 2014, former German ambassador Wolfgang Ischinger said, while noting that it would now be unthinkable for President Vladimir Putin to be invited back into the club of leading industrialized nations.
Russia was excluded from the group months after its annexation of the Ukrainian peninsula of Crimea. Moscow is also accused of supporting pro-Russian separatists in eastern Ukraine.
"The G7 group is not in a position to resolve big international crises alone," Ischinger said. "Neither the Ukraine crisis nor the Syria conflict can be resolved without Russia."
To invite Putin back into the group now would be "a confession of a serious defeat," said Ischinger, who chairs the influential Munich Security Conference, an annual gathering of world leaders and foreign ministers.
But he said the door should "stay open in the long term" for Russia's return, he said.
The leaders of Britain, Canada, France, Germany, Italy, Japan and the United States will hold their annual G7 meeting in the Japanese province of Ise-Shima on Thursday and Friday.
Putin, meanwhile, is due to visit EU and NATO member Greece while the summit is underway.
"For a long time we did not want to accept that the idea of convergence between Russia and the West under Putin has become an illusion," Ischinger said of the G7 summit.
He noted, however, that the group's common values allowed them to achieve more than larger formats such as the G8 or the G20.